(1) Field of the Invention
The present invention relates generally to non-destructive testing and, more particularly, to a sensor array for non-destructively monitoring a structure to detect a critical structural event and calculate the location of the critical structural event.
(2) Description of the Prior Art
The performance of modern-day military helicopters, missiles, tanks, aircraft, and other static or dynamic structures is critically dependent on the reliability of advanced composite materials and heterogeneous armor materials. There has been a reluctance to deploy such high performance materials in critical structural applications because of their susceptibility to in-service damage. The damage occurring in these materials may be difficult to track and can propagate quickly during operation of the vehicle or structure, resulting in the loss of the entire vehicle.
Conventional non-destructive evaluation techniques are labor intensive, expensive, error prone, and unworkable for efficient integration into composite and heterogeneous structures. Autonomous integrated Structural Health Monitoring (SHM) techniques are a revolutionary concept in the maintenance of structures. SHM techniques continuously monitor the condition of a structure. Various approaches for SHM under development use piezoceramic sensors and actuators that require separate wiring connections for each sensor and actuator element, storage of pre-damage data for each sensor, and instrumentation for active generation and sensing of diagnostic signals. When the structural geometry is complex—e.g., either the structure has varying thickness, curvature, ribs, joints, or heterogeneous materials, or damage is located near boundaries of the structure—it becomes difficult to detect small damage using SHM methods. In addition, the number of sensor circuits and computations required increases the overall complexity and cost of the structure.
One approach to this problem is to integrate many fiber-optic strain gauges directly within the structural material. An optical fiber with twenty or more Bragg gratings can measure static and dynamic strains at discrete locations on the structure. An optical analyzer can multiplex over each fiber and each grating to measure strains at a large number of points on a structure. This approach is being implemented on bridges, pressure tanks and other structures. However, fiber optic sensors have limitations when applied to monitoring complex composite structures where damage can occur anywhere on the structure and in any direction. For example, discrete strain measurements can miss damage because the measurement is very localized at the fiber/grating. In addition, an optical analyzer using multiplexing and multiple connections is expensive; measurements are not simultaneous and the frequency bandwidth may be too low to sense Acoustic Emission (AE) signals.
AE sensors are presently suitable for detection of damage at “hot spots.” The use of AE measurements for SHM of large structures may have certain advantages since it is a passive sensing technique. Passive sensing methods are simpler and may be more practical than using active interrogation methods. However, present passive acoustic emission and monitoring techniques require bulky instrumentation with numerous channels, long connections, and centralized data analysis. It may be impractical to embed these systems on the structure to operate in the field. Another limitation is that AE waveforms from such sensors are too complicated for purposes of source characterization.
U.S. Pat. No. 6,399,939 issued Jun. 4, 2002 to Sudaresan et al. discloses a sensor array apparatus and method for reducing the number of sensors and instrumentation channels required, by an order of magnitude, while retaining the sensitivity in the high frequency range to detect incipient damage in the structure. The disclosure of this patent and its cited references is hereby incorporated by reference in its entirety.
Thus, there remains a need for a new and improved system for non-destructively monitoring a structure to detect a critical structural event using a single channel continuous acoustic emission sensor that provide sufficient spatial coverage to efficiently sense AE signals while, at the same time, may be used to calculate the location of the critical structural event.